Intervertebral discs are soft and compressible. They are interposed between adjacent vertebral body elements of the spine. They act as shock absorbers for the spine, allowing it to flex, bend, and rotate. Degenerative disc disease can occur throughout the spine, but most often occurs in the discs in the lower back (lumbar region) and the neck (cervical region).
As the process of degeneration continues, micro tears or cracks occur in the outer layer (annulus fibrosus) of the disc. The jellylike material inside the disc (nucleus pulposus) may be forced out through the tears or cracks in the annulus, which causes the disc to bulge, break open (rupture), or break into fragments.
The economic impact of degenerative disc disease is enormous accounting for a significant morbidity and lost wages.
The physical properties of the disc are the nucleus pulposus which is composed of type II collagen and the annulus fibrosis which surrounds the disc and gives it significant form. The annulus composed of type I collagen. The nucleus pulposus is largely made up of molecules called proteoglycans. These proteoglycans have an affinity for water. It is this retention of water and the stoichiometry of folded molecules that is responsible for the unique mechanical properties of the disc. If these proteoglycans are depleted, the discs become more rigid and the loss of fluid results in a disc that is thinner and less compliant. Clinically this results in narrowing of the distances between the vertebral elements. This is best seen on magnetic resonance imaging. Typically discs have a bright signal on T2 pulse-weighted sequences and they are hypointense on corresponding T1 images. This is due to the high fluid content of the discs. As the disc loses fluid i.e. the loss of proteoglycans, the disc loses its water signal and becomes anhidrotic and eventually mineralizes. As a result, these individuals develop the symptoms in the spine contributable to loss of the normal disc architecture. As the process of degeneration continues, one develops micro tears or cracks and fissures in the annulus fibrosis and through these cracks and fissures the nucleus pulposus, which is largely gelatinous, may extrude. The extruded disc material may efface the dura and cause significant nerve compression which may result in traumatic neuritic pain and or motor loss.
Once the damage to the disc is so complete the ability to correct the problem is limited to artificial implants to restore the disc space. A more traditional approach was to use a spinal fusion implant that provided the spacing between the vertebral bodies, but thereafter allow bone growth to fuse the adjacent vertebrae together destroying any ability of these fused vertebrae to articulate.
More recently, cervical prosthetic discs have been proposed for the cervical repairs in particular ones that do not fuse the vertebral bodies, but instead allow a limited range of motion. These new articulating implant devices are a better choice until scientists can perfect disc tissue regeneration and natural biologic repair of the nucleus pulposus.
The present invention as described hereinafter is an improved spinal implant design that enhances mobility and articulation in a self-aligning and reliable construction.